a. Field of the Invention
The present invention relates, in general, to methods for manufacturing membranes for electro-acoustic transducers and membranes made by the disclosed methods, more particularly, to a method of manufacturing a membrane for an electro-acoustic transducer such as a micro speaker, miniature microphone or receiver, for use in mobile communication devices, the membrane made from an elastomer using spray coating.
b. Background Art
It is desirable for electro-acoustic transducers used in microelectronics to be as compact as possible. However, especially in the case of speakers, it is also desirable that the speaker should be able to output in the broadest range of frequencies possible.
A speaker comprises a membrane attached to a voice coil, which is positioned within a magnetic field defined by a permanent magnet and yoke arrangement. The performance of the speaker is dependent on the resonant frequency. Above the resonant frequency, the output response is relatively flat. Therefore, a low resonant frequency gives rise to a wideband performance. The resonant frequency is a function of the stiffness and the mass of the moving parts. The stiffness is dependent on two factors: the stiffness of the membrane and the stiffness of the back volume.
The membrane in conventional micro speakers comprises a thermoplastic foil formed by deep drawing or stamping. The foil has a relatively high stiffness and, conversely, a relatively low compliance. Without additional measures, such a membrane will cause a relatively high resonant frequency in a conventional micro speaker.
Recent improvements in membrane technology has seen membranes comprised of an elastomer having stiffness significantly less than conventional thermoplastic foils. For example, one such improvement is detailed in U.S. patent application Ser. No. 13/380,428, filed Jun. 23, 2010 (PCT date), and published as U.S. Pat. Publ. No. 2012/0093353 A1 on Apr. 19, 2012. (The entire disclosure of such application is herein incorporated by reference.) The disclosed membrane is comprised of an elastomer, including silicone, and is made using injection molding.
While using injection molding to form elastomer membranes has many advantages over the use of deep drawing or stamping thermoplastic foil, it is not without its drawbacks. In particular, injection molding requires a two-part mold which adds to the complexity of the process and limits many aspects of the design, such as the minimum thickness obtainable for the membrane. The two-part mold also may not always be perfectly aligned, creating non-uniformity among multiple parts being produced. And even when near-perfect alignment can be achieved with new molds, during use, the mold pieces are susceptible to wear, which can cause alignment problems later.
Conventional membranes of thermoplastic foils are often constructed of multiple layers of different materials. The different layers of a multi-layer membrane can have different mechanical properties, allowing for adjustments for stability, damping or other performance characteristic. Examples of multi-layer membranes and the particular benefits of such are described in U.S. Pat. No. 8,284,964 and U.S. patent Ser. No. 14/699,548, filed on Apr. 19, 2015, the disclosures of each of which are herein incorporated by reference in their entirety. However, multi-layer membranes comprised of an elastomer are not possible using injection molding. Further, while injection molding typically provides less variations in the thickness of a finished membrane than the deep drawn or stamping process, there is still a desire to improve the uniformity of the thickness of the membrane in order to lesson or eliminate tumbling of the membrane caused by asymmetries.
There is thus the need therefore to improve the process of manufacturing a membrane comprised of an elastomer for use with an electro-acoustic transducer.